This invention relates generally to techniques of mass storage in computer systems, and more specifically, to the optimization of such systems with regard to characteristics such as power consumption, physical ruggedness, access time, and the like.
The most common computer mass storage device currently in use is a hard disk drive. As is well known, one or more rotating disks include magnetic storage material coated on at least one, and usually both sides, of each disk. At least one magnetic read/write head is provided for each disk side and made to be radially movable across its surface as the disk is rotated at a uniform speed. Data is magnetically recorded on the disk surfaces on concentric tracks. Each track is divided around its circumference into several sectors, each sector storing a block of data of a predetermined size, most commonly 512 bytes plus overhead information related to the block of data. A given sector is addressed by specifying its disk drive, accessing head, cylinder and sector number. The cylinder and head information, in effect, together designate a tract on a disk surface where the designated cylinder intersects the surface.
Hard disk drives are capable of storing a large amount of information, in the order of hundreds of megabytes of data, in a small package and at a low cost. Disk drives have their disadvantages, however, such as a slow access time relative to that of solid state random-access-memory ("RAM"), being subject to a limited life because of wear on mechanical elements, having a low tolerance for mechanical shock relative to that of solid state memories, and requiring significantly greater amounts of power in order to rotate the disk and move the read/write heads.
As a result, non-volatile solid-state memory systems are being introduced into the market place to take the place of part or all of the disk storage capacity of a mass storage system in particular applications. The technology being utilized is flash electrically erasable and programmable read-only-memory ("flash EEPROM"). With this technology, arrays of memory cells are provided wherein each cell is formed of a transistor having at least a portion of its gate electrically floating, the charge on the floating gate thus controlling the conduction of the transistor and its detectable state. Arrays of tens of megabytes of such cells are being provided in very small packages, and the capacity is increasing. The power requirements of flash EEPROM memory are very low, they are physically rugged, have no mechanical parts which wear out, and in overall operation provide faster average access time when compared with disk drive memory. However, flash EEPROM memory is currently much more expensive per byte than disk storage, thus limiting its application to situations where its advantages are very important.
It is a principal object of the present invention to provide a mass storage system which combines both disk and Flash EEPROM systems in a manner which takes advantage of the strengths of each type of memory.
It is a further object of the present invention to provide such mass storage for use in a computer without the need of any significant alteration in the computer system operating software system.